According to WebMD, there are two types of intrauterine devices in the United States, and each works differently. The hormonal IUD releases a hormone to thicken cervical mucus and prevent sperm reaching the egg. A non-hormonal IUD is placed to prevent sperm from reaching the egg but does not need hormones to achieve this goal.
WebMD notes that the hormonal form of IUD can last for three or five years. The three-year IUD is called Skyla and the five-year IUD is called Mirena. Both release the hormone progesterone, which thickens cervical mucus. As sperm must pass through the cervix to meet an egg and form a zygote, this prevents fertilization from happening. According to Mayo Clinic, this T-shaped contraceptive also thins the lining of the uterus, which prevents any fertilized egg from attaching and progressing to a pregnancy.
WebMD states that the copper IUD lasts for 10 years, and it acts as a barrier to prevent fertilization. Mayo Clinic highlights that the presence of copper in the IUD causes an inflammatory reaction in the uterine lining. This is toxic for sperm, which reduces the risk of fertilization. In addition, the thinned uterine lining makes it difficult for a fertilized egg to attach and progress to a pregnancy.